Verb
(idiomatic) To talk excessively.
(idiomatic, derogatory) To speak in a disrespectful manner about somebody.
Source: en.wiktionary.orgI've never really considered doing stand up, but I have done readings/spoken word things fairly often in which I'll just tell a bunch of stories and run off at the mouth. I'm a big tangent person. Laurie Notaro
I used to always run off at the mouth and talk about people. I just didn't know that it would make a living for me. D. L. Hughley